1. Field of the Invention
The present invention relates to an active matrix type EL display device with display pixels including an electroluminescence element (hereinafter referred to as an EL element) and a thin film transistor arranged in a matrix form, and particularly to an art for stably illuminating each display pixel by preventing voltage drops in capacitance lines connected to, and shared by, the display pixels.
2. Description of the Related Art
EL elements have various advantages, including, because they are self illuminating elements, an obviated need for a backlight as required in liquid crystal display devices and unlimited viewing angle. Because of these advantages, it is widely expected that EL elements will be use in the next generation of display devices.
Two basic methods are known for driving EL elements. One of these is called a simple, or passive, matrix type, with the other, which employs a thin film transistor as a switching element, is known as an active matrix type. The active matrix type does not suffer from cross talk between the column and row electrodes, which is a problem known in the simple matrix type. Moreover, because the EL elements are driven with a lower current density, a high luminescence efficiency can be expected.
FIG. 3 is a circuit diagram schematically showing an active matrix type EL display device. In the figure, the display pixels GS1, GS2, GS3, . . . GSj are arranged in one row. One display pixel GS1 includes an organic EL element 11, a first thin film transistor 12 (an N channel type transistor) acting as a switching element in which a display signal DATA1 is applied to the drain and which is switched on and off in response to a select signal SCAN, a capacitance 13 which is charged by the display signal DATA1 supplied when the first thin film transistor 12 is switched on and which maintains a maintenance voltage Vh when the first thin film transistor 12 is switched off, and a second thin film transistor 14 (a P channel type transistor), with its drain connected to a drive supply voltage Vdd and its source connected to the anode of the organic EL element 11, for driving the organic EL element when the maintenance voltage Vh is supplied from the capacitance 13 at the gate.
The other display pixels GS2, GS3, GSj have an equivalent structure. Although the display pixels are also arranged in the column direction, this arrangement is not shown in the figure in order to simplify the drawing. Reference numeral 15 represents a gate signal line which is connected to and shared by each of the display pixels GS1, GS2, GS3, . . . GSj for supplying a select signal SCAN. Reference numeral 16 represents a gate drive circuit for supplying the select signal SCAN to the gate signal line. Reference numeral 17 represents a capacitance line which is connected to and shared by the capacitance 13 of each of the display pixels.
The select signal SCAN becomes H level during a selected one horizontal scan period (1H), and the first thin film transistor 12 is then switched on based on the select signal. Next, a display signal DATA1 is supplied to one end of the capacitance 13 and the capacitance 13 is charged with a voltage Vh corresponding to the display signal DATA1. The voltage Vh is maintained in the capacitance 13 for a period of one vertical scan period (1V) even after the first thin film transistor 12 is switched off due to the select signal SCAN becoming L level. Because this voltage is supplied to the gate of the second thin film transistor 14, the second thin film transistor 14 becomes continuous in response to the voltage Vh and the organic EL element 11 is illuminated.
However, in larger size conventional EL display devices, differences in luminance throughout the display device have been observed.
The capacitance line 17 is formed from chrome evaporated on a glass substrate, in consideration of heat endurance and ease of processing. Because the capacitance line 17 is extended on the display region in order to be connected to and shared by each of the display pixels GS1, GS2, GS3, . . . GSj, a resistance and a floating capacitance are inevitably generated. For example, in an active matrix type EL display device having a number of pixels of 220×848, the resistance value of one capacitance line 17 is approximately 320 Ω and the floating capacitance is approximately 20 pf. The resistance and floating value increase as the number of pixels increases.
The capacitance line 17 must be kept constant because it acts as a reference potential for charging the display signal DATA1. However, when the resistance value of the capacitance line 17 is large, the potential of the capacitance line 17 becomes unstable when the active matrix type EL display device is driven, causing a problem that the EL element 11 is not illuminated at a luminance corresponding to the display signal DATA1. In other words, a select signal SCAN having an H level is supplied to the gate signal line 15 based on the select signal SCAN and the display signal DATA1 is supplied to one end of the capacitance 13. This causes the display signal DATA1 to be applied to the capacitance 13 and the capacitance 13 is charged. If the resistance of the capacitance line 17 is large, the potential would vary.